Variable radiation length bubble chamber



' Dec. 12, 1967 M. GOLDHABER 3,358,144

VARIABLE RADIATION LENGTH BUBBLE CHAMBER Filed Dec. 21, 1965 INVENTOR M-AURICE GOLDHABER BY WWW United States Patent ABSTRACT OF THE DISCLOSURE A- hydrogen bubble chamber having neon added to vary selectively its radiation length. The latter can be varied in the range of 30 to 540 cm. A circulating system for the liquid mixture during filling and adding to the chamber avoids phase separation of the neon and hydrogen.

The invention described herein was made in the course of, or under, a contract with the United States Atomic Energy Commission.

The present invention relates to a dual purpose bubble chamber utilizing neon-hydrogen mixtures for the detection and tracking of ionized particles.

The conception of the bubble chamber in 1952 in which a superheated liquid was used to display the tracks of ionizing particles made it possible to record rare nuclear events more readily than had been possible up to that time. This was a result of the higher stopping power of the liquid in the bubble chamber as compared to cloud chambers wherein lower density materials are used.

The first bubble chambers were of very small size, beginning with those of only a few hundred cubic centimeters in volume (about three inches along a typical dimension) to the eighty inch bubble chamber now in use at the Brookhaven National Laboratory, Upton, NY.

As the energies of particles under study still continue to climb, consideration is being given to even larger bubble chambers, such as a 14-foot chamber, in which to accommodate the nuclear events to be recorded. Larger bubble chambers are required because, as the energies to be studied increase, it takes a longerdistance in a particular medium before the particles will come to rest or for 'a particular interaction to occur with a finite probability.

The earliest bubble chambers contained ordinary (diethyl) ether, and in time, several other materials have 'been used successfully, including liquid hydrogen, liquid nitrogen, and isopentane, among others. As the density of a liquid is about a hundred times or more than that of 'the corresponding gas in a cloud chamber, it is readily volume while at the same time not compressing the reactions into such a short distance which would hamper their examination and study.

With the increasing energy of ionizing particles being 3,358,144 Patented Dec. 12, 1&67

studied, however, it is becoming necessary to enlarge substantially the size of the hydrogen bubble chamber to accommodate and study properly the consequent reactions. The cost of scaling up already large hydrogen installations rises precipitiously and this is causing increased concern by those who are charged with the responsibility of providing the necessary facilities but at the same time must operate within stringent financial limitations. 1

It has been proposed that a bubble chamber containing heavy liquids, such as propane, could be used for tracking interactions occuring at some of the higher energy levels, and indeed such chambers are available. However, such a chamber has limited usefulness as it cannot be used with liquid hydrogen due to differences in pressure, temperature and other operating conditions and all of the other special equipment required. Hence, a chamber using such heavy liquids would not be useful where it is necessary or desirable for other reasons to have a hydrogen medium so that a hydrogen bubble chamber would still be required. In addition, heavy liquids have a tendency to produce interactions which are not suitable for some investigations.

The present invention makes it possible to provide a bubble chamber for use with higher energy particles without scaling up further the existing hydrogen bubble chamber, and at the same time avoiding the drawbacks associated with the use of some of the heavy liquids.

In accordance with this invention neon is added to hydrogen for use in a bubble chamber to reduce the radiation length of the bubble chamber liquid so that increased energies of particles may be accommodated without having to increase the size of the bubble chamber. Neon is so similar to hydrogen in its pertinent physical characteristics that no significant change in the bubble chamber is required except for adding the facilities to add and withdraw the neon as desired and to make provision for mixing. An additional and important advantage of this invention is that by varying the proportion of neon contained in the hydrogen it is possible to adjust the radiation length of the mixture over a wide range of values. This compares with an'infiexible radiation length of centimeters for pure propane and 1100 cm. for pure hydrogen. By radiation length is meant a measure of the mean conversion distance for gamma rays in .the particular medium. This distance is a measure of how far the gamma rays must travel to produce electron-positron pairs as a result of interaction with the nuclei of the bubble chamber liquid. The radiation length of a medium, therefore, indicates the ranges-of particles which can be contained in a given volume, so that, in effect, the instant invention makes it possible to have a bubble chamber of variable radiation length to accommodate a wide variety of experiments without expenditure of additional time and money for elaborate special arrangements or need to build a bubble chamber of different size to accommodate a new series of experiments. Other advantages of this invention include the possibility that the bubble chamber can be used for conducting neutrino experiments merely by filling it with pure neon.

It is thus a first object of this invention to provide an improved bubble chamber utilizing mixtures of neon and hydrogen.

A further object of the invention is a methodof operating a bubble chamber by varying the amounts of neon added to hydrogen to obtain desired sensitivity to the presence of ionizing particles. I q i Another object is the use of neon-hydrogen mixtures to obtain a desired radiation length in a bubble chamber.

Still another object of the invention is a bubble chamber of selectively variable stopping power.

Other objects and advantages of this invention will hereinafter become apparent from the following description of preferred modes of carrying out this invention taken with the accompanying figure illustrating a hydrogen bubble chamber.

As is well understood in the art, a bubble chamber consists of a vessel containing a liquid which is made sensitive to ionizing radiation by heating it to a temperature above that of its boiling point. The ionizing particles entering the liquid produce ions which act as nuclei for the production of small bubbles of vapor tracing out the paths traversed by the particles. These paths are photographed and in this way a permanent record of the tracks is made. The greater the stopping power of the liquid in the chamber the greater will be the frequency of events which will be detected and recorded in the liquid. On the other hand, however, the use of liquid with an excessive stopping power for a particular type of experiment will result in compression of the event to an extent which tends to obscure the shape and nature of the tracks. Thus it is desirable to have as long a stopping power as is possible in order to show completely and clearly what has hapened consistent with such practical considerations as the limitation of equipment size and frequency of events. Of course, as the stopping power is reduced the frequency with which events will occur drops olf so that there is a practical limitation based upon this consideration.

When a bubble chamber is operated it is usually done in a pulsed or intermittent manner in order to clear all bubbles away prior to each exposure to ionizing particles. Thus, typically, after each exposure,v the chamber would be subjected to added compression so that the boiling temperature is raised momentarily to clear away or condense the vapor in the bubbles. As a practical matter, the pressure would be dropped to sensitize the liquid just prior to an expected dosage of ionizing particles.

In the use of hydrogen in a bubble chamber there is the added problem of maintaining cryogenic conditions as the chamber will be operated and maintained at temperatures down to about K. and pressures up to about 150 p.s.i.a. A typical expansion ratio would be a 0.6% increase in volume to reduce pressure to sensitize the liquid hydrogen.

In accordance with this invention it has been found that the addition of liquid neon in controlled amounts to the hydrogen in a bubble chamber makes it possible to effect significantly and controllably certain very important characteristics of the bubble chamber. First, it is possible and feasible to reduce the radiation length of the liquid to accommodate ionizing particles of increased energy without producing reactions tending to obscure reactions under study. Further, the addition of neon makes it possible to select almost at will the mentioned characteristics of the bubble chamber thereby rendering the latter more effective as an instrument of research. Thus, if it is desired to increase the frequency of events to be detected by compressing the events into shorter distances, this can be done conveniently and relatively quickly by adjusting the concentration of neon in the bubble chamber. This type of control, that is, selecting the stopping power of the bubble chamber, is totally lacking in the conventional bubble chamber. In addition, the pressure and temperature characteristics of neon as it relates to liquefaction and handling aresimilar enough to hydrogen so that ity is totally compatible with hydrogen for this type of application.

In a series of experiments which were run to verify and demonstrate this concept, an existing hydrogen bubble chamber was utilized by adding liquefied neon' to the hydrogen in the chamber. This required only a tank of neon and a pipe with a valve and an instrument to measure the amount of neon added. The volumes of neon were obtained from the measured amounts of the materials present in the chamber. Mixing of the two liquids in the chamber was observed visually and was enhanced by circulating some of the mixture out of the chamber and back again at another location through existing pipes.

Referring to the figure, there is shown a twenty-inch bubble chamber 10 enclosed in a vacuum tank 12. Chamber 10 consists of an expanded lower portion 14 which may be square in cross-section and a narrower upper portion 16 which may be circular in cross-section, connected by a converging transition section 18. A piston 22 controlled by a rod 24 extending out of chamber 10 is mounted in upper portion 16 as illustrated, while a piston ring 24 maintains a certain amount of piston sealing. As is understood in the art, the space below piston 22 is filled with the liquid while the space above piston 22 is filled with the vapor of the liquid below. Friction produced by ring 24 and seepage of liquid produces the vapor. Piston 22 is raised to cause the expansion and lowered to produce the compression mentioned above in the operation of bubble chambers. One or more windows 39 covered with suitable transparent material permits observation and photography of the events occurring within the chamber 10. Sampling lines 32, 34, 36 and 38 were used to obtain samples of the chamber contents to insure proper mixing and also to verify the composition of the liquid. Pressure devices 42, 44 and 46 were utilized to measure the pressure within the chamber 10.

An inlet pipe 52 controlled by valve 54 was used for filling chamber 10. In order to maintain uniform mixing of the neon and helium during filling operations a tube 56 opening into the gas space above piston 22 was used which communicated to inlet pipe 52 through a mixing valve 62. During normal operation of chamber 10 valve 62 was kept closed. However, during filling or adding to chamber 10, valve 6 2 was opened. Piston 22 would be reciprocated so that each upward movement would build up pressure in the gas space and result in vapor passing by way of tube 56 and pipe 52 into chamber 10. In view of the seepage up past piston 22 this in effect resulted in an overall circulation of the fluid and caused good overall mixing within chamber 10.

For each experiment run in bubble chamber 10, measurements were taken to determine the volumes of neon and hydrogen mixed therein and various temperatures were established for each mixture to determine for each mixture of neon and hydrogen the temperatures to avoid phase separation of the neon and hydrogen within the chamber. A fuller discussion of this phase separation is discussed in the paper entitled Liquid Vapor Equilibrium in the System Neon-Hydrogen by Street and Jones submitted for publication in the Journal of Chemical Physics. Normally, the bubble chamber would not be operated when the neon-hydrogen mixture is separated into phases although certain types of experiments of course would be possible because tracking can still occur in each phase.

The following examples illustrate this invention.

A 20" bubble chamber such as is illustrated in the figure, previously in use solely with hydrogen, was utilized to carry out this invention. A positive displacement pump (not shown) supplying the liquid neon and the known dimensions of the chamber provided the data to calculate the volumetric proportions of neon and hydrogen at any given time. Sampling lines 32-38 were used to verify uniform mixing of neon and hydrogen and pipes 52, 56 and 58 were provided to circulate the liquids to insure thorough mixing after additional neon was introduced each time. The following table is a sampling of tests obtained with neon in the range of 2.6 to atomic percent indicating that in such a range of neon concentration it was possible effectively to vary the radiation length from about 30 cm. to 540 cm. with relative ease. An examination of photographs taken of known interactions for known particles entering the chamber demonstrated that no unexpected or confusing reactions took place which would tend to obscure results.

TABLE Atomic, Chamber Chamber Stroke, Expansion Radiation Percent Pressure, Temp., inch Ratio, Length, neon p.s.i.a. K. Percent cm.

151 36. 6 0. 57 30 113 31. 4 0. 7 6 33 156 32. V; 0. 57 35 120 29. 7 0. 57 40 127 29. 7 Ms 0. 66 50 149 30. 94s 0. 85 67 143 30. 0 1. 13 90. 5 113 28. 3 1.13 116.5 149 30. 3 916 0. 85 165 117 28. 3 A 1. 13 171 95 27. 6 0. 94 269 63 26. 0 Ma 0. 85 306 2.6 61 25.2 V; 0.94 540 It is quite apparent from the data presented above that it is possible by this invention to obtain in a bubble chamber any particular radiation length in the range of about 30 cm. to 540 cm. as required in a particular series of experiments. Thus it is apparent that the hydrogen bubble chamber has been made into a more flexible tool than heretofore seemed possible and furthermore extends substantially the range of energies to be applied without the necessity of increasing the size of the bubble chamber accordingly.

While only preferred embodiments of this invention have been described it is understood that many variations thereof may be made without departing from the principles thereof and that the invention is to be defined only by the appended claims.

I claim:

1. A bubble chamber having variable radiation length comprising a sealed container, means for maintaining hydrogen a liquid in said chamber, means for expanding the contents of said container an incremental amount to place said liquid contents in a superheated condition, and means to maintain said chamber full with variable selected amounts of liquid neon with said hydrogen.

2. The bubble chamber of claim 1 in which said neon is maintained at a concentration to obtain a preselected radiation length in the range of about cm. to 540 cm. for the mixture of hydrogen and neon in said bubble chamber.

3. The bubble chamber of claim 1 including means to insure uniform mixing of said hydrogen and neon.

4. In a method of operating a hydrogen bubble chamber consisting of means forming a sealed chamber and means for selectively varying the volume of the space within said chamber, the improvement comprising the steps of supplying preselected amounts of liquid hydrogen to said chamber, maintaining said hydrogen in a liquid condition, adding a preselected amount of liquefied neon to establish a desired radiation length for said bubble chamber, and cyclically compressing and expanding said contents to render the hydrogen-neon mixture sensitive to ionizing radiation.

5. The method of claim 4 in which the neon-hydrogen within said chamber is circulated to enhance mixing, as additional liquid is added to said chamber.

References Cited UNITED STATES PATENTS 2,900,518 8/1959 Good 250-83 RALPH G. NILSON, Primary Examiner.

ARCHIE R. BORCHELT, Examiner.

A. B. CROFT, Assistant Examiner. 

4. IN A METHOD OF OPERATING A HYDROGEN BUBBLE CHAMBER CONSISTING OF MEANS FORMING A SEALED CHAMBER AND MEANS FOR SELECTIVELY VARYING THE VOLUME OF THE SPACE WITHIN SAID CHAMBER, THE IMPROVEMENT COMPRISING THE STEPS OF SUPPLYING PRESELECTED AMOUNTS OF LIQUID HYDROGEN TO SAID CHAMBER, MAINTAINING SAID HYDROGEN IN A LIQUID CONDITION, ADDING A PRESELECTED AMOUNT OF LIQUEFIED NEON TO ESTABLISH A DESIRED RADIATION LENGTH FOR SAID BUBBLE CHAMBER, AND CYCLICALLY COMPRESSING AND EXPANDING SAID CONTENTS TO RENDER THE HYDROGEN-NEON MIXTURE SENSITIVE TO IONIZING RADIATION. 